The present invention relates to the manufacture of novel honeycomb structures from plasticized batches of inorganic or organic powders.
Ceramic and metallic honeycomb structures are widely used for applications such as catalyst substrates, honeycomb heaters, and the like, and the fabrication of such structures from plasticized batches of inorganic powders is well known. U.S. Pat. No. 3,320,044 to Cole describes a method of constructing ceramic honeycombs from sheets or ribbons of plasticized ceramic material, while U.S. Pat. Nos. 3,790,654 and 3,905,743 to Bagley describe direct extrusion methods and apparatus for such manufacture.
The more widely used extrusion methods for forming honeycomb structures commonly involve compounding a batch comprising inorganic powders together with added plasticizer, vehicle and binder components to achieve a plastic consistency. The plasticized batch is thereafter forced through an extrusion die to form a honeycomb shape which is then solidified by drying, heat-curing, reaction sintering or similar processing.
Dies for honeycomb extrusion typically comprise a die body incorporating a plurality of feedholes on an inlet face which extend through the body to convey the plasticized batch material to a discharge section on an opposing die outlet face. The discharge section incorporates a criss-crossing array of discharge slots, cut into the outlet face to connect with the feedholes within the die body, these slots reforming the batch material supplied by the feedholes into the interconnecting cell wall structure of the desired cellular honeycomb structure.
As the uses for such honeycomb structures have increased, so also has the need for providing more finely structured honeycombs. A fundamental limitation of the extrusion approach, however, is the fact that neither the feedholes nor the discharge slots in conventional extrusion dies may be multiplied without limit. Real limits on the cell density (the number of cells per unit honeycomb cross-section) and cell wall thicknesses obtainable by extrusion through honeycomb extrusion dies are imposed by available die machining methods. Also limiting are the finite strength and stiffness of available die fabrication materials. Die extrusion pressure increases with increasing cell density due to higher shear rates from thinner slits and due to increased die friction drag area. Thus it remains difficult to provide ceramic honeycombs of very fine dimensions for various specialty applications where conventional honeycomb dimensions are too large.
Also of interest for a variety of fluid processing applications are honeycombs offering channels of other than straight cylindrical or polygonal cross-sectional shape. U.S. Pat. Nos. 5,393,587 and 5,633,066, for example, disclose honeycomb designs offering curved or twisting flow paths through a channeled structure, for purposes such as controlling flow rates, enhancing fluid-wall contact, and the like. However, the cell and cell wall dimensions of these honeycombs remain relatively large, and maintaining precise control over the size, shape and direction of the channels forming the flowpaths is difficult.
It is therefore one object of the present invention to provide novel processes for the manufacture of cellular honeycomb structures from plasticized powder batch materials that can offer products of much finer cell structure and thinner cell walls than can be produced through the direct extrusion of honeycombs from conventional plasticized powder batch materials.
It is another object of the invention to provide novel designs for honeycomb structures, and methods for making them, including curved, conical, or other functionally graded honeycomb shapes offering new capabilities for the controlled conveyance and/or treatment of fluid streams arising within catalytic treatment or other chemical processing environments.
Other objects and advantages of the invention will become apparent from the following description thereof.